Resins, such as acrylic resins, polyesters, polycarbonates, cellulose derivatives, polyvinyl alcohol, and polyimides, are highly transparent, lighter than inorganic glass, and easy to mold. Thus, these resins are widely used in the fields of optical technology and information equipment relevant to recording, display, and transmission of information; for example, in optical disks, optical films for liquid crystal display devices and organic electroluminescent display devices (hereinafter also referred to as “organic EL display devices”), optical lenses, and optical fibers.
Such a resin has a hydrogen bond donor (e.g., the hydrogen atom of a hydroxy group or the hydrogen atom of an amido group) or a hydrogen bond acceptor (e.g., the carbonyl oxygen atom of an ester group or a nitrogen atom contained in an aromatic heterocyclic ring), and thus the resin forms a hydrogen bond with water to adsorb water (hereinafter the resin will be referred to as “hygroscopic resin”). The resin may therefore absorb water in association with a change in environmental humidity over time, leading to variations in dimensions and properties, including mechanical properties, such as rigidity and strength, electrical properties, such as resistivity, and optical properties, such as refractive index.
A variation in retardation is one of the problems caused by adsorption of water on a resin. A retardation film is used for increasing the viewing angle of a liquid crystal display device or preventing external light reflection in an organic EL display device. The retardation of the film is sensitive to the amount of water adsorbed on the resin, because the retardation depends on the birefringence of the film; i.e., the difference between the refractive index in the direction of molecular orientation and that in the direction orthogonal thereto.
Although acrylic resins, polycarbonates, or cellulose derivatives are used as hygroscopic resins for formation of retardation films, the cellulose derivatives, which have high water adsorption (high moisture content), cause large variations in retardation in association with changes in environmental humidity over time.
In recent years, liquid crystal display devices or organic EL display devices have been increasingly used for large-size and high-definition applications, such as television sets, and retardation films have accordingly been demanded to have higher quality. Liquid crystal display devices or organic EL display devices for large-size and high-definition applications are required to be used under severer conditions than conventional ones. Thus, retardation films used in these display devices are demanded to exhibit a small humidity-dependent variation in optical performance.
In view of these situations, several techniques have been proposed which involve incorporation of a specific additive into an optical film for reducing a humidity-dependent variation in optical performance.
PTL 1 discloses a cellulose ester film containing a polyester and a polyhydric alcohol ester or an aromatic-terminal ester.
PTL 2 discloses a cellulose ester film containing a compound having a specific value; i.e., quotient of the molecular weight of the compound divided by the total number of hydrogen bond donors and hydrogen bond acceptors of the compound.
PTL 3 discloses a cellulose ester film containing a highly hygroscopic compound exhibiting a difference between moisture contents determined under different conditions of 2% or more.
The present inventors have evaluated the cellulose ester films disclosed in PTLs 1 to 3 under severer conditions than conventional ones. Consequently, the present inventors have found that although these cellulose ester films exhibit some advantageous effects, the films need further improvements for use in recent display devices for high-definition applications.
A film used in such applications has been demanded to exhibit no variation in performance even under such a severe condition involving direct exposure of the film to water resulting from condensation during its conveyance. The present inventors have found that the conventional techniques disclosed in PTLs 1 to 3 are less effective under the severe condition; i.e., direct exposure to water.